Spectral Interrogating for Absolute OPD Demodulation
Absolute OPD demodulation is a universal method for readout of an FFP sensor, by which higher measurement accuracy could be obtained, compared with the intensity demodulation. As mentioned above, the measurand loaded on the FFP sensor is modulated spectrally; therefore, the interrogators are usually meant to measure the spectra and then convert the results to measured data . Thus, getting the spectrum of the signal light is crucial. There are several common methods to obtain full spectra of FFP sensors including broadband light source combining with s pectrometer, broadband light source combining with tunable filter, and swept or tunable laser combining with photodetector.
Spectral Interrogation Based on Spectrometers
The general schematic for full spectra acquiring of an FFP sensor using a spectrometer is demonstrated in Figure 5.4. The FFP sensor is illuminated by a broadband light source. The reflected light from
Figure 5.5 Operation schematic of diffraction gratings.
the sensor is received by a spectrometer. The light source could be SLED, SLD, or Erbium-doped fiber ASE light source, whose coherent lengths are generally shorter than the OPDs of FFP sensors.
A typical spectrometer is diffraction grating combining with photodetectors array [9,10]. Volume phase grating (VSP) is a kind of diffraction grating, like a conventional optical device used to spatially separate the different wavelengths or colors contained in a beam of light . The device consists of a collection of diffracting elements (narrow parallel slits or grooves) separated by a distance comparable to the wavelength of light under test, as given in Figure 5.5. The VSP spectrometer is a highly integrated device and easy to mass produce, and has been successfully commercialized as a spectral interrogator for FFP sensors. The operating diagram of using VPG to measure the full spectrum of an FFP sensor is schematically shown in Figure 5.6 .
Figure 5.7 Schematic diagram of fiber Fourier transform spectrometer.
Another method to interrogate the spectra of FFP sensors is to use Fourier transform spectroscopy (FTS), as shown in Figure 5.7. The system uses a fiber-optic Michelson interferometer with one arm wrapped on a piezoelectric-driven stretcher capable of inducing a similar to 10-cm fiber length change to sample the spatial domain data. The spectrum can be obtained by Fourier transformation of the sampled data. Passive polarization compensation is utilized to eliminate the possibility of random polarization fading in the interferometer causing apodization of the interferogram. FTS is an efficient analytical tool for the analysis of broadband spectra with high sensitivity and reasonable resolution [11,12]. However, complex structure, low measuring speed, large volume, and high price limit its applications.